1. Field of Use
This invention relates to the construction of high-strength light-weight hollow boom sections for multi-section telescopic crane booms.
2. Description of the Prior Art
Mobile cranes are required to have telescopic booms which can handle increasingly heavier loads and raise them to greater heights. It is known to increase boom sizes for load-handling capability, and increase strength by enlarging the physical size of such telescopic booms. As the boom sections increased in size, and as the lengths to which they can be extended are increased, the booms become extremely heavy and more difficult to operate. Various approaches in boom design and construction have been employed to achieve greater size and strength without suffering undue weight penalties. For example, lattice-type booms are sometimes employed and attempts have been made to lighten the boom by piercing holes in the heavy gauge sheet steel of which some boom sections are fabricated. Also, designs employing optimum selection and arrangement of boom section components have been employed to achieve high strength versus weight ratios.
A constant effort has been made to improve a cantilever boom design in the hydraulic crane industry, the emphasis being on the optimum strength. State regulations in the United States impose a weight limit on the vehicles travelling the roads and highways. Construction industry, on the other hand, demands higher reach and lift capacities. Prior art designs show that several manufacturers have developed crane booms with thin side plates and with vertical or longitudinal stiffeners trying to reduce the weight to keep the larger cranes roadable without a special permit. Most common booms, however, have been rectangular four-plate type in cross-section. This design is economical and quite adequate for smaller cranes; however, when the increase capacity and longer booms are required, the height of the boom sections will increase with increase in section properties. At increasing heights, thin side plates without stiffeners have to be considerably thicker in order to protect against shear buckling, thus adding to the weight of the boom.
Typical prior art boom sections comprise rectangular, trapezoidal or triangular cross sections. Very often, in sections wherein the top, bottom and side walls are made of steel plate, high strength structural steel about 3/16 of an inch thick is the minimum thickness in the walls to obtain acceptable performance in respect to shear buckling, tensile, compressive stresses. To further increase the shear buckling capabilities of the boom sections, stiffeners are generally welded to the boom walls. To accommodate sliding between the adjacent boom sections, bearing pads of various materials and configurations are used, such as rollers, bearings, Teflon slide of bearing pads. For practical design reasons, pads are limited in size. And, since the bearing loads are high and the load bearing surface is small, bearing pads are subject to high compressive stresses. Therefore, high wear occurs and thus adjustment means are required to maintain proper clearance between the boom sections.
An example of the prior art is shown in U.S. Pat. No. 4,112,649 issued Sept. 12, 1978 to Fritsch, and which has been assigned to an assignee common with the present invention. That patent utilized longitudinal stiffeners which were bent up fabrications and located along the upper and lower edges of the side walls only. Vertical side stiffeners were generally U-shape or welded to each side of the boom section and also welded to the inner edges of the corner stiffeners. The top and bottom walls were plain and simply abutted against the side walls and their stiffeners.
Other examples of the prior art are the U.S. Pat. Nos. 3,789,563 of Feb. 5, 1974 which discloses the use of two opposed rigid metal extrusions forming upper and lower sides of the boom securely locked in interengagement with side walls; 4,016,688 of Apr. 12, 1977 which discloses the use of right angle steel panels for the purpose of reinforcing the plates which form the sides of the boom; 4,045,936 of Sept. 6, 1977 which shows I-beam side walls for a boom having a thin web with stiffeners for reinforcement along the edges trusses and stiffeners are located interiorly of the beam flanges; and 4,171,598 of Oct. 23, 1979 which utilizes corner angles formed by a rectangular corner member which is overlapped by a double wall.
The West German Pat. No. 2,205,093 of 1973 shows a number of hollow box sections having lengthwise protrusions from the side webs and by which it is supported on rollers. The protrusions are situated on their respective webs in the tension zones above the neutral axis. The East German Pat. No. 31498 of 1964 also shows various corner reinforcements for web sections in a boom.